1. Field of the Invention
This invention relates to a random copolymer, a crosslinked product thereof, and a composition comprising the crosslinked product.
The random copolymer according to the present invention is a novel copolymer comprising monomer units of a non-conjugated diene having a specific structure. A crosslinked product of the random copolymer, and a composition comprising the crosslinked product have high tensile strength and high elasticity even when their degree of crosslinking or gel fraction is high, and also exhibit high resistance for abrasion.
2. Description of the Related Art
Polyolefins have excellent properties from the viewpoint of practical use, so that they have been widely used as they are, or as a mixture of two or more kinds of polyolefins. In addition, modified polyolefins such as a polyolefin which has grafted thereinto an unsaturated organic acid or a derivative thereof, or an unsaturated aromatic monomer, and a crosslinked polyolefin, are also employed in a wide variety of fields.
Of the polyolefins, polypropylene resins including propylene homopolymers or polypropylene and propylene copolymers such as a block or random copolymer of propylene with ethylene, and polyethylene resins including ethylene homopolymer or polyethylene, and ethylene copolymers such as a copolymer of ethylene with propylene and a copolymer of ethylene with propylene and butene are of considerable utility.
In particular, attention has been focused on crosslinked products of polypropylene resins because of their advantageous properties such as of high rigidity, high melting points, glossiness, transparency, and excellent electric characteristics.
However, poly(.alpha.-olefin) resins other than polyethylene resins are known in the art to have problems, as polypropylene resins typically show such that the resins contain a hydrogen atom bonded to the tertiary carbon atom of each recurring monomer unit thereof, and, accordingly, a radical crosslinking method such as a method in which a peroxide or radiation is employed to induce a crosslinking reaction is not applicable to obtain a crosslinked product of poly(.alpha.-olefin) resins. This is because cleavage of the main chain of the poly(.alpha.-olefin) resin occurs prior to a crosslinking reaction when the above radical crosslinking method is applied. For this reason, the resulting crosslinked product is not practically usable, and cannot be processed into a highly expanded or compressed article.
The use of a crosslinking aid is therefore essential to obtain crosslinked products of poly(.alpha.-olefin) resins such as a polypropylene resin, and various proposals have hitherto been made on the crosslinking aid which is effective to obtain a crosslinked product of a polypropylene resin. Examples of such methods include, for instance, a method disclosed in Japanese Patent Publication No. 45-23035 in which a rubber such as polybutadiene rubber, natural rubber or a polyisoprene rubber is blended, a method disclosed in Japanese Patent Publication No. 53-28060 in which a liquid 1,2-polymer of butadiene is blended, and a method disclosed in Japanese Laid-Open Patent Application No. 53-138470 in which a syndiotactic 1,2-polymer of butadiene, is blended.
However, the crosslinked products of polypropylene resins obtained by the above conventional methods are remarkably poor in resistances for heat, ozone and weather. This is because the above-described crosslinking aid contain a double bond in their main chains, or the tertiary carbon atom in the main chain thereof is in the allyl position. Moreover, the above crosslinking aids are gummy, and a polypropylene resin mixed with any of the crosslinking aids, the resulting mixture is to have poor mechanical properties of such as rigidity, and the advantages of the polypropylene resin which inherent has high rigidity are thus impaired.
In order to solve the above problems, proposals have been made as disclosed in Japanese Patent Publication No. 64-2139, and Japanese Laid-Open Patent Applications Nos. 58-210930, 58-210931 and 61-252248. These proposals are based on the use of copolymers of an .alpha.-olefin and an 1,4-diene, and crosslinked product thus obtained overcome, to a certain extent, the drawbacks in the prior art. However, such crosslinking or crosslinked products are still insufficient in the crosslinking efficiency, tensile strength, elasticity and resistance for abrasion.
Japanese Laid-Open Patent Application No. 59-155416 discloses a method for preparing a block copolymer, in which a non-conjugated diene represented by the following formula is indispensably employed: ##STR2## wherein a is an integer of from 1 to 10, and R.sup.1, R.sup.12 and R.sup.13 each independently represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. A crosslinked product of the block copolymer obtained by the above method has impact resistance. However, it may have low transparency, so that it would not be utilized without limitation. The above polymer may also be disadvantageous in that the foaming conducted simultaneously with the crosslinking would not give a crosslinked foam product having cells which are uniform in size.
Japanese Laid-Open Patent Application No. 2-51512 discloses a random copolymer of ethylene, an .alpha.-olefin having 3 to 12 carbon atoms, and a non-conjugated olefin represented by the following formula: ##STR3## wherein b is an integer of from 2 to 10, and R.sup.14 represents an alkyl group having 1 to 8 carbon atoms, and R.sup.15 and R.sup.16 each independently represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, the ethylene and the .alpha.-olefin being bonded with each other in a molar ratio of (5-90)/(95-10), and the content of monomer units of the non-conjugated diene contained in the copolymer being such that the iodine value of the copolymer is 3 or higher; and a composition comprising 100 parts by weight of the above random copolymer, 0 to 300 parts by weight of a reinforcement, 0 to 150 parts by weight of a softening agent, and 0.1 to 10 parts by weight of a crosslinking agent. The Specification of the above Patent Application, however, simply describes that the random copolymer can improve the vulcanization speed of the composition comprising it.
The inventors of the present invention have proposed a method for preparing an improved copolymer of propylene, 6-methyl-1,6-octadiene and/or 7-methyl-1,6-octadiene, and optionally ethylene as disclosed in U.S. Pat. No. 4,987,193. Specifically, the method is such that an unsaturated copolymer of propylene and the branched diene which is 6-methyl-1,6-octadiene and/or 7-methyl-1,6-octadiene, and optionally ethylene, is produced by a two-step process wherein in a first polymerization step homopolymerization of propylene is conducted until the propylene polymer amounts to 0.01 to 10% by weight of the total and final copolymer, and in a second polymerization step the random copolymerization of propylene, 6-methyl-1,6-octadiene and/or 7-methyl-1,6-octadiene, and optionally, ethylene is then conducted as a continuation of the first step polymerization.